The present invention relates to a process for making a lube base stock from olefin-containing feedstocks having lower molecular weights than the lube base stock, using more than one oligomerization zone. Included in this invention is a process for making predominately bright stock lube base stock.
Lubricant oils of high viscosity and high oxidation stability are desirable. Such materials can be prepared by hydrocracking, hydroisomerizing and otherwise hydroprocessing various wax fractions and by polymerizing normal alpha olefins such as 1-decene. The former route has the advantage of moderate costs, but the oxidation stability is not exceptional. As attempts are made to improve the oxidation stability by increasing the severity of the hydroprocessing steps, the yield of lube declines, as does its viscosity. The latter route gives an exceptionally stable product, but suffers the disadvantage of high cost. It would be desirable to provide new processes for generating high viscosity and highly stable products. The present invention provides such a process.
U.S. Pat. No. 6,025,533 to Vora et al. (xe2x80x9cOligomer Production with Catalytic Distillationxe2x80x9d) teaches production of heavy oligomers (C7+ oligomers) from C4 paraffins and olefins by a combination of dehydrogenation and oligomerization. The process has at least one catalyst bed in the top of a distillation column for separating the oligomerization effluent of the dehydrogenation and oligomerization combination.
U.S. Pat. No. 5,276,229 to Buchanan et al. (xe2x80x9cHigh VI Synthetic Lubricants From Thermally Cracked Slack Waxxe2x80x9d) teaches oligomerizing alpha-olefins produced from thermal cracked slack wax.
U.S. Pat. No. 5,015,361 to Anthes et al. (xe2x80x9cCatalytic Dewaxing Process Employing Surface Acidity Deactivated Zeolite Catalystsxe2x80x9d) teaches oligomerization of propylene in two stages using ZSM-23 and ZSM-5 to form a low pour point, high cloud point product, followed by dewaxing.
U.S. Pat. No. 4,855,524 to Harandi et al. (xe2x80x9cProcess For Combining the Operation of Oligomerization Reactors Containing a Zeolite Oligomerization Catalystxe2x80x9d) teaches combining the operation of a primary reactor that oligomerizes a C3-7 feed to gasoline range hydrocarbons and a high pressure secondary reactor that oligomerizes the effluent of the first reactor to make distillate or lubes.
U.S. Pat. No. 4,678,645 to Chang et al. (xe2x80x9cConversion of LPG Hydrocarbons to Distillate Fuels or Lubes Using Integration of LPG Dehydrogenation and MOGDLxe2x80x9d) teaches converting C4- paraffins to higher hydrocarbons by the combination of catalytic or thermal dehydrogenation of a paraffinic feedstock to produce olefins and conversion of olefins to gasoline and distillate boiling range materials in a low pressure oligomerization catalytic reactor and a high pressure oligomerization catalytic reactor.
U.S. Pat. No. 4,608,450 to Miller (xe2x80x9cTwo-Stage Multiforming of Olefins to Tetramersxe2x80x9d) teaches a two-stage process for preparing a C3 or C4 olefin tetramer using nickel-containing HZSM-5 zeolite catalyst.
A variety of patents disclose catalysts useful for oligomerization.
U.S. Pat. No. 5,453,556 to Chang et al. (xe2x80x9cOligomerization Process For Producing Synthetic Lubricantsxe2x80x9d) teaches an oligomerization process using a catalyst having an acidic solid with a Group IVB metal oxide modified with an oxyanion of a Group VIB metal.
U.S. Pat. No. 5,270,273 to Pelrine et al. (xe2x80x9cOlefin Oligomerization Catalystxe2x80x9d) teaches an olefin oligomerization catalyst having a supported, reduced Group VIB metal oxide on an inorganic support, such as MCM-41.
U.S. Pat. No. 5,243,112 to Chester et al. (xe2x80x9cLubricant Range Hydrocarbons From Light Olefinsxe2x80x9d) teaches oligomerizing an olefinic feedstock over a medium pore zeolite catalyst (HZSM-22).
U.S. Pat. No. 5,171,909 to Sanderson et al. (xe2x80x9cSynthetic Lubricant Base Stocks From Long-Chain Vinylidene Olefins and Long-Chain Alpha- and/or Internal-Olefinsxe2x80x9d) teaches oligomerization of long-chain olefins using certain acidic montmorillonite clay catalysts.
U.S. Pat. No. 5,146,022 to Buchanan et al (xe2x80x9cHigh VI Synthetic Lubricants From Cracked Slack Waxxe2x80x9d) teaches oligomerizing with a Lewis acid catalyst a mixture of C5-C18 or C6-C16 alpha-olefins produced from thermal cracking of slack wax.
U.S. Pat. No. 5,080,878 to Bowes et al. (xe2x80x9cModified Crystalline Aluminosilicate Zeolite Catalyst and Its Use in the Production of Lubes of High Viscosity Indexxe2x80x9d) teaches oligomerization with a modified zeolite (ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-38, or ZSM-48).
U.S. Pat. No. 4,962,249 to Chen et al. (xe2x80x9cHigh VI Lubricants From Lower Alkene Oligomersxe2x80x9d) teaches oligomerization of lower olefins with a reduced valence state Group VIB metal oxide on porous support. In one embodiment, a feedstock of lower olefins is contacted with surface deactivated, acidic, medium pore, shape selective metallosilicate catalyst under oligomerization conditions, then reacting the mixture with ethylene in contact with an olefin metathesis catalyst under metathesis conditions, then oligomerizing the metathesis product in contact with a reduced valence state Group VIB metal catalyst on porous support.
U.S. Pat. No. 4,542,251 to Miller (xe2x80x9cOligomerization of Liquid Olefin Over a Nickel-Containing Silicaceous Crystalline Molecular Sievexe2x80x9d) teaches oligomerization in the liquid phase using nickel-containing silicaceous crystalline molecular sieve catalysts to produce lube base stock.
U.S. Pat. No. 4,417,088 to Miller (xe2x80x9cOligomerization of Liquid Olefinsxe2x80x9d) teaches oligomerization of liquid olefins using intermediate pore size molecular sieves to produce lube base stock.
EP 791,643 A1 (xe2x80x9cLubricating Oilsxe2x80x9d) teaches a process for the production of lubricating oils having a viscosity index of at least 120 and a pour point of xe2x88x9245 C. or less by oligomerizing a feedstock comprising one or more C5-18 1-oleflins in the presence of an oligomerization catalyst comprising an ionic liquid.
In conventional hydrodewaxing, the pour point is lowered by selectively cracking the longer chain wax molecules, mostly normal and slightly branched paraffins. A disadvantage associated with catalytic dewaxing is that the wax is degraded to lower molecular weight materials. For example, waxy paraffins may be cracked down to butane, propane, ethane and methane and so may branched paraffins which do not contribute to the waxy nature of the oil. It is desirable to limit the degree of cracking which takes place during a catalytic dewaxing process, because these lighter products are generally of lower value than the higher molecular weight materials, and because the viscosity index and oxidation stability of the resulting oil is degraded by the loss of paraffins.
A major breakthrough came with the discovery of new dewaxing catalysts, which were found to isomerize rather than crack the wax molecules. Isomerization alters the molecular structure of wax molecules, and generally decreases the pour point of a molecule without significantly changing its boiling point. In contrast to wax cracking, isomerized molecules are retained in the lubricating oil base stock, increasing yield of lubricating oil base stock without reducing viscosity index or oxidation stability significantly.
U.S. Pat. No. 5,135,638 to Miller (xe2x80x9cWax Isomerization Using Catalyst of Specific Pore Geometryxe2x80x9d) discloses a process for producing lube oil from a feedstock having greater than 50% wax. The feedstock is isomerized over a catalyst comprising a molecular sieve (e.g., SAPO-11, SAPO-31, SAPO-41, ZSM-22, ZSM-23, and ZSM-35) and at least one Group VIII metal at a pressure of from about 15 psig to about 2000 psig.
U.S. Pat. No. 5,246,566 to Miller (xe2x80x9cWax Isomerization Using Catalyst of Specific Pore Geometryxe2x80x9d) discloses a process similar to that of U.S. Pat. No. 5,135,638, but where the waxy feed has a pour point of above about 0 C. and contains greater than about 70% paraffinic carbon.
U.S. Pat. No. 5,282,958 to Santilli et al. (xe2x80x9cUse of Modified 5-7 xc3x85 Molecular Sieves For Isomerization of Hydrocarbonsxe2x80x9d) discloses isomerizing a feed including straight chain and slightly branched chain paraffins having 10 or more carbons using an intermediate pore size molecular sieve (e.g., SAPO-11, SAPO-31, SAPO-41, ZSM-22, ZSM-23, and ZSM-35). Feeds which may be processed by this method include waxy feeds, which contain greater than about 50% wax.
U.S. Pat. No. 5,082,986 to Miller (xe2x80x9cProcess for Producing Lube Oil From Olefins By Isomerization Over a Silicoaluminophosphate Catalystxe2x80x9d) discloses a process for making a C20+ lube oil from olefins or reducing the pour point of a lube oil comprising isomerizing the olefins over a catalyst an intermediate pore size silicoaluminophosphate molecular sieve and at least one Group VIII metal.
Large pore zeolites represent another class of catalysts that have been taught for wax isomerization.
EP 464,546 to Degnan et al. (xe2x80x9cProduction of high viscosity index lubricantsxe2x80x9d) teaches producing a high viscosity index lubricant from a petroleum wax feed having a paraffin content of at least 40 weight percent. The catalyst is a low acidity zeolite isomerization catalyst having an alpha value of below 20. Zeolite Beta, which contains boron as a framework component of the zeolite, is taught as being preferred.
WO 96/26,993 to Apelain et al. (xe2x80x9cWax Hydroisomerization Processxe2x80x9d) teaches for producing a high viscosity index lubricant catalytically dewaxing waxy paraffins by isomerization in the presence of hydrogen and a low acidity large pore zeolite isomerization catalyst having a ratio of SiO2/Al2O3, as synthesized, of at least 200:1.
WO 96/13,563 to Apelain et al. (xe2x80x9cWax Hydroisomerization Processxe2x80x9d) teaches an isomerization process for producing a high viscosity index lubricant using a low acidity large pore molecular sieve having a crystal size of less than 0.1 micron, an alpha value of not more than 30 and containing a noble metal hydrogenation component.
EP 225,053 to Garwood et al. (xe2x80x9cLubricant Production Processxe2x80x9d) teaches isomerization dewaxing using a large pore, high silica zeolite dewaxing catalyst, followed by a subsequent dewaxing step which selectively removes the more waxy n-paraffin components. The selective dewaxing step may be either a solvent or a catalyst dewaxing, preferably using a highly shape selective zeolite such as ZSM-22 or ZSM-23.
EP 659,478 to Perego et al. (xe2x80x9cProcess for Preparing Amorphous, Catalytically Active Silico-sluminasxe2x80x9d) teaches a process for producing a high VI lubricant from a waxy hydrocarbon feed by isomerization in the presence of hydrogen and a low acidity large pore molecular sieve.
Non-zeolitic catalysts are also taught for wax isomerization.
U.S. Pat. No 5,049,536 to Belussi et al. (xe2x80x9cCatalytically Active Silica and Alumina Gel and Process For Preparing Itxe2x80x9d) catalysts are described based on silica and alumina gel and their use in isomerization processes.
EP 582,347 to Perego et al. (xe2x80x9cCatalyst for the Hydroisomerization of Long-chain N-paraffins and Process for Preparing itxe2x80x9d) teaches a bifunctional catalyst for hydroisomerization. That catalyst has at least one Group VIIIA noble metal on a calcined amorphous silica and alumina gel.
U.S. Pat. No. 5,723,716 to Brandes et al. (xe2x80x9cMethod For Upgrading Waxy Feeds Using a Catalyst Comprising Mixed Powdered Dewaxing Catalyst and Powdered Isomerization Catalyst Formed Into A Discrete Particle (LAW082)xe2x80x9d) teaches combinations of zeolitic and non-zeolitic catalyst components.
U.S. Pat. No. 6,008,164 to Aldrich et al. (xe2x80x9cLubricant Base Oil Having Improved Oxidative Stabilityxe2x80x9d) teaches a method of producing a lube base stock by separating, into a plurality of fractions based on molecular shape, a hydroisomerized hydrocarbon wax, and collecting the fractions that have a preselected oxidative stability.
U.S. Pat. Nos. 4,417,088; 4,542,251; 4,608,450; 4,678,645; 4,855,524; 4,962,249; 5,015,361; 5,049,536; 5,080,878; 5,082,986; 5,135,638; 5,146,022; 5,171,909; 5,243,112; 5,246,566; 5,270,273; 5,276,229; 5,282,958; 5,453,556; 5,723,716; 6,008,164; and 6,025,533 are hereby incorporated by reference for all purposes.
The present invention provides a process for preparing a lube base stock from a lower molecular weight olefinic feedstock via oligomerization. The process involves separating an olefinic feedstock in a first separator into fractions that include at least a light olefin fraction and a medium olefin fraction. The light olefin fraction is contacted with a first oligomerization catalyst in a first oligomerization zone to produce a first product having increased molecular weight. The product of the first oligomerization is combined with the medium olefin fraction and the combined olefins contacted with a second oligomerization catalyst in a second oligomerization zone to produce a second product having increased molecular weight. The second product is then separated in a second separator into a light byproduct fraction and a heavy product fraction, wherein the heavy product fraction can be used as a lube base stock.
Preferably, the olefinic feedstock includes at least 10% olefins, more preferably at least 30% olefins, most preferably at least 50% olefins. The boiling point of the olefinic feedstock is greater than 180xc2x0 F., preferably greater than 258xc2x0 F., more preferably within the range of from 2580 to 1100xc2x0 F., most preferably within the range of from 258xc2x0 to 650xc2x0 F.
Preferably, the boiling point of the light olefin fraction is no more than 350xc2x0 F., more preferably in the range of from 50xc2x0 to 350xc2x0 F. Preferably, the boiling point of the medium olefin fraction is in the range of from 250xc2x0 to 650xc2x0 F.
In one embodiment, the fractions coming off the first separator further comprise a waxy heavy fraction (preferably having boiling points of at least 650xc2x0 F.). In that embodiment, the waxy heavy fraction is thermally cracked to produce addition olefins, which are separated in a third separator into an additional light olefin fraction and an additional medium olefin fraction. Preferably, the additional light olefin fraction is sent to the first oligomerization zone and the additional medium olefin fraction is sent to the second oligomerization zone.
In some embodiments, the olefinic feedstock is derived in whole or in part from the dehydrogenation of a paraffinic feedstock. The dehydrogenation can produce diolefins, which are preferably selectively hydrogenated to reduce at least a portion of the diolefins to monoolefins. The product from the first oligomerization zone may also include diolefins, which may also be selectively hydrogenated.
In one embodiment, the olefinic feedstock is produced by a Fischer-Tropsch process, either directly from the Fischer-Tropsch process or by dehydrogenation of a highly paraffinic feedstock produced by a Fischer-Tropsch process.
Preferably, the olefinic feedstock is purified to remove oxygenates and other impurities. One purification method is by hydrotreatment of that highly paraffinic feedstock. If hydrotreatment is used for purification, the hydrotreated olefinic feedstock should be dehydrogenated to replace olefins lost by the hydrotreatment process. An alternative purification method is by adsorption with acid clay. Preferably, the olefinic feedstock is dehydrated and decarboxylated to convert any alcohols or acids which may be present to olefins.
Skeletal isomerization can be used to adjust the pour and cloud point of the final product to a desired value. Skeletal isomerization can be induced at any of a number of points in the process, including (1) on an olefinic feedstock while it is being hydrotreated, (2) while the hydrotreated olefinic feedstock is being dehydrogenated, (3) in the first oligomerization zone, (4) in the second oligomerization zone, (5) while hydrofinishing the product of the second oligomerization zone, or (6) while hydrofinishing the heavy product fraction. Preferably, skeletal isomerization is induced prior to the oligomerization zone.
The first oligomerization catalyst can be the same or different as the second oligomerization catalyst. In one embodiment, the oligomerization catalysts are an inorganic oxide or a Group VIII metal on an inorganic oxide support, more preferably a Group VIII metal on a zeolitic support. The oligomerization catalysts can be nickel on ZSM-5. In an alternative embodiment, the oligomerization catalysts can include an acidic ionic liquid.
Preferably, the product from the second oligomerization zone has a number average molecular weight at least 10% higher than the olefinic feedstock, more preferably at least 20% higher than the olefinic feedstock. Preferably, the product from the second oligomerization zone is hydrofinished prior to the separation step, and/or the heavy product fraction is hydrofinished.
Preferably, at least a portion of the light byproduct fraction is recycled either to the first oligomerization zone, the second oligomerization zone, both the first and second oligomerization zones and/or to the second separator.
In one embodiment, the oligomerization zone is located within a catalytic distillation unit used to both produce the product and separate the product into a light byproduct fraction and a heavy product fraction. In that embodiment, the olefinic feedstock can also be contacted with an oligomerization catalyst in a fixed bed prior to the catalytic distillation unit. Preferably, at least a portion of the light byproduct fraction is recycled either to the catalytic distillation unit or to the fixed bed or to both the catalytic distillation unit and the fixed bed.
Preferably, the heavy products fraction has a viscosity of greater than 2 cSt at 100xc2x0 C., a viscosity index of at least 80 and a pour point of less than xe2x88x9210xc2x0 C. More preferably, the viscosity index is at least 120 and a pour point of less than xe2x88x9220xc2x0 C. More preferably, heavy products fraction is separated into at least one of the following fractions:
a) a light lube base stock fraction having a viscosity of from 2 to 7 cSt at 100xc2x0 C.;
b) a heavy lube base stock fraction having a viscosity of from 6 to 20 cSt at 100xc2x0 C.; and
c) a bright stock fraction having a viscosity of greater than 180 cSt at 40xc2x0 C.
More preferably, the heavy product fraction is predominately a bright stock fraction having a viscosity of greater than 180 cSt at 40xc2x0 C.
The production of lube base stock can be maximized by recycling substantially all of the light byproduct fraction, either to the first oligomerization zone, the second oligomerization zone, both the first and second oligomerization zones or to the second separator.